The present invention relates to a process for fabrication of dielectric optical waveguide devices by utilizing molecular beam epitaxial growth in order to fabricate semiconductor compounds and to form thin films of the mixed crystals thereof.
The prior art processes for fabricating three-dimensional thin-film waveguide devices may be divided into three types. In the first type, a thin film having a high refractive index is formed over the surface of a substrate having a low refractive index, and an etching mask having a desired pattern is placed upon the thin film so that the undesired portions of the thin film are etched out to form the waveguides.
According to the second type, a thin film having a high refractive index is formed over the surface of a substrate having a low refractive index, and guide patterns having a refractive index slightly lower than the refractive index of the thin film are placed upon the thin film so that the regions immediately below the guide patterns may have an apparent increased refractive index.
The third type is a selective epitaxial process in which a suitable shadow mask is used to interrupt one or more of the molecular beams from being applied to the surface of a substrate.
In general, in the waveguides the interfaces must be finished with an extremely high degree of accuracy in order to minimize the loss due to the scattering of light. However, according to the first or etching process, it is most difficult to finish the side faces of the waveguide with a desired high degree of accuracy, and the structure and optical properties of the waveguide are very sensitive to the dimensional accuracy of the side faces. Therefore it is extremely difficult to attain the desired structure and optical properties of the waveguides.
In the waveguides fabricated by the second type of process, the light energy is substantially propagated in the regions immediately below the guide patterns. Therefore accuracy of dimension of the side faces is not so severe, but the side faces must be finished with the same degree of accuracy with that of the side faces of the waveguides fabricated by the first or etching process if it is desired to couple the light signals transmitted through the different waveguides.
The third type of process has a distinct advantage over the first and second types in that three-dimensional embedded waveguides can be formed by a series of epitaxial growth steps. However selective epitaxial growth utilizes the shadows of a mask which are cast on different regions on the surface of the substrate due to the difference in incidence angle between the molecular beams. Therefore, except for straight waveguides, it is difficult to form a waveguide in a complex configuration or a plurality of waveguides which are arrayed in parallel with each other.
In addition, the above prior art processes have a common defect that the top surfaces of waveguide devices are irregular; that is, they are not flat. That is, two-dimensional waveguides may be formed, but the three-dimensional waveguide devices in which a plurality of two-dimensional waveguides are overlayed one upon another cannot be fabricated.